Blades for wind turbines are typically constructed of glass-reinforced plastics (GRP) on a sub-structure, which may be formed of wood, glass fibre, carbon fibre, foam or other materials. Graphite fibre in epoxy resin is also used. The plastics resin can be injected into a mould containing the sub-structure to form the outer surface of the blade. The blade may also be built up as a series of layers of fibre material and resin. In some cases, the fibre material is pre-impregnated with resin.
A typical wind turbine blade may have a length of between 20 and 60 metres or more. As the interior of the blade is generally hollow, a “floor” is provided within the blade proximate the hub-engaging end of the blade. The blade floor is a bulkhead about 0.5 metres to 2.5 metres into the blade that prevents service personnel falling into a blade while working in the hub.
It is known, for example from U.S. Pat. No. 4,297,076, to provides the blades of a wind turbine with strain gauges and to adjust the pitch of portions of the blades in response to the bending moment on the blades measured by the strain gauges. Optical fibre strain sensors are known and WO 2004/056017 discloses a method of interrogating multiple fibre Bragg grating sensors forming an array along a single fibre. In the system of WO 2004/056017, Bragg gratings are defined in the optical fibre at spaced locations along the optical fibre. When the optical fibre is put under strain, the relative spacing of the planes of each Bragg grating changes and thus the resonant optical wavelength of the grating changes. By determining the resonant wavelength of each grating, a strain measurement can be derived for the location of each grating along the fibre. Optical strain sensors operating on the principle of back scattering which do not require discrete gratings along the fibre are also known.
The weight of turbine blades made on the same production line varies slightly from blade to blade. Consequently, the three turbine blades that typically make up a single turbine are carefully selected to have similar weights to ensure the turbine is properly balanced. Most optical fibre strain sensor systems are wavelength division multiplexed (WDM). Each sensor in the same array is identified in the reflected signals from the sensor array by its wavelength. Each sensor must therefore have a different wavelength at all times from other sensors in the same array. If the sensors in each of the three blades of a wind turbine are to be processed as a single array, each blade must contain sensors with resonant wavelengths in different wavelength bands so that each sensor of the complete, three blade array always has a characteristic wavelength. This imposes limitations on the selection of turbine blades in that the blade must be selected both to have the appropriate weight for a balanced turbine and the appropriate wavelength of sensors to prevent ambiguity in the sensor signals. Consequently, the blades of a typical optical fibre sensor system might be categorised into three types, e.g. A, B and C, each having a distinct set of sensor wavelengths. Each turbine requires one of each type of blade for the sensor system to operate correctly, and this significantly complicates blade selection for balancing turbines and maintaining an appropriate inventory of spare blades.
This invention, at least in its preferred embodiments, seeks to provide a scheme for implementing optical fibre sensors in turbine blades in a way that allows any blade to be selected with any other blades for use together on a turbine. It also allows a single spare blade to be used to replace any other blade should the need arise.